Application for the grant of a patent on this type of device has been filed under U.S. Ser. No. 638,354. There, a canister-like water reservoir is provided with two adjoining threaded connections, one of which is used for receiving a cartridge-shaped flow-through filter insert and the other, usually kept closed, is for refilling with water. In actual use the filter container with the filter insert facing downwardly is disposed on top of a catch reservoir which may have a spout on the side from where the filtrate can be removed.
An air compensation hole is located at the seal of the threads of the filter insert or below or next to the filter, which allows for the flow of air into the container as soon as the pressure in the container has been sufficiently reduced by the outflow of filtrate. In this disposition of the air compensation hole at a middle height in relation to the filter or lower than the latter, the hydrostatic pressure which decisively determines the flow through the filter is low. This may cause stoppage of the flow through the filter, so that filtrate no longer emerges.
Measurements have shown that the hydrostatic pressure at the level of the air compensation hole fluctuates slightly around the value of the ambient air pressure. Sometimes blockage of the filter results in air entering the filter, and this in turn can result in oxidation and/or contamination by microbes of the filter material. Because of the balanced hydrostatic pressure, practically no pressure acts on the filter, so that its flow resistance can hardly be overcome. Thus a slight flow through the filter mainly occurs on account of the capillary suction effect of the gaps in the filter material, the width of which is strongly dependent on the respective arrangement and type of the material filled into the filter. Since many types of different filters are commercially available, the respective filter determines how quickly the filtrate emerges; no action which might make possible an adaptation to different types of material t be filtered and different degrees of soiling is provided.